Large marine vessels are usually steered by large hydraulically operated rudders. This, of course, is nothing new, and some type of hydraulic rudder has been in existence at least since the patent to Christensen, U.S. Ser. No. 641,192, Jan. 6, 1906. However, these systems have been evolving into more complexed rudder assemblies which, in turn, rely more heavily on increasingly complex safety mechanisms. These large rudder assemblies, if they suffer a loss in hydraulic fluid, can render an large vessel completely helpless and at the mercy of the prevailing environmental forces. Furthermore, when there is a complete loss of hydraulic fluid, the rudder assembly may uncontrollably lash back and forth creating an extremely hazardous condition for those persons who are in the immediate vicinity of the device.
Safety mechanisms on large ocean-going vessels use systems that replenish the hydraulic fluid to the ram cylinders. For example, see U.S. Pat. Nos. 4,209,986; 3,949,696; and 3,913,517.
Other attempts at providing a secure hydraulic system are: blocking valves to block the hydraulic fluid in the ram cylinders (see Reliability Improvements for Vessel Steering Systems, page 6-1, U.S. Dept. Comm./NTIS, J. J. Henry Company, Incorporated, New York), or some type of mechanical stop, as shown in U.S. Pat. No. 3,818,853, and Norwegian Patent No. 33,429.